Apparatus for measuring changes in radial and/or axial position of a rotor in a drive system including an emf producing stationary conductor

ABSTRACT

An apparatus for measuring change of position of a rotor in a drive system having a drive rotor and a driven rotor generally concentrically mounted with respect to the drive rotor, the drive and driven rotors being magnetically coupled together. The apparatus includes a conduction device stationarily mounted between the rotors and a device for measuring the EMF (Electro Motive Force) produced in the conduction device as the rotors rotate. The apparatus also includes a device for determining, from the measurement, the relative position of the two rotors and/or the position of one of the rotors.

The present invention relates to apparatus for measuring the change ofposition of a rotor.

We will describe the invention with reference to socalled seal-lesspumps and the like in which a driven inner rotor is connected to animpeller of the pump, the impeller and inner rotor being sealinglymounted within a closed system through which the fluid to be pumpedflows, the inner rotor being mounted within a generally cylindricalsealed shroud which closely surrounds said inner rotor, and there beingmounted a drive outer rotor surrounding the shroud which is magneticallycoupled with the inner rotor so that as the drive outer rotor rotates,it drives the driven inner rotor and hence the impeller.

The magnetic coupling may comprise permanent magnets forming part of theinner and outer rotor, or there may be provided coils which cooperate bymeans of magnets produced by eddy currents, or the outer rotor may, inplace of permanent magnets comprise electro magnets, the inner and outerrotor effectively forming an electric motor so that a separate motor isnot required.

Such a drive system may be used to operate other apparatus for examplemixers, fans and blowers. Furthermore, arrangements are known where thedrive and driven rotors are disc-like and arranged face to face.

One of the difficulties of such an arrangement, as is well known, isthat, because the inner rotor is mounted within the sealed shroud and istherefore not visible, it is not possible to check the bearings of theinner rotor without dismantling the apparatus. This has been aparticular problem and requires the pump to be stripped at regularintervals.

The present invention relates to means for measuring change of position(e.g. radial and/or axial position) of a rotor in a drive systemcomprising a drive (usually outer) rotor, a driven (usually inner) rotorgenerally concentrically mounted with respect to the drive rotor, thedrive and driven rotors being magnetically coupled together, saidapparatus comprising conduction means stationarily mounted between saidrotors, and means for measuring the emf (electro motive force) producedin said conduction means as said rotors rotate, and means fordetermining, from said measurement, the relative position of the tworotors and/or the position of one of the rotors.

It should be understood that if the bearings on the driven rotor wear,the relative radial positions of the two rotors will change and thiswill affect the magnetic flux coupling between the two rotors, wherebythe emf produced by the conduction means will vary.

The conduction means is preferably mounted on a stationary housingbetween the driven and drive rotor.

The drive system may be a seal-less pump which includes a sealing shroudbetween the drive rotor and the driven rotor, said conduction meansbeing mounted on or forming part of said sealing shroud.

Whilst the conduction means may comprise a single length of conductivewire which cuts the lines of the magnetic field between the drive anddriven rotors as they rotate relative to the wire, in a preferredarrangement, two conduction means may be provided spaced at, forexample, 180° intervals around the axis of the housing. In a preferredarrangement one conduction means may be at the top of the housing andone at the bottom. In a yet more preferred arrangement, four conductionmeans may be provided spaced at substantially 90° intervals around theaxis of the rotors.

In place of a single length of wire, where the rotors are inner andouter coaxial rotors, the conduction means may comprise one (or more)loops (each loop comprising a sensor) including lengths extendingsubstantially parallel to the axis ("active conductors"), and otherlengths extending circumferentially around the housing ("connectorwires"). The lengths extending parallel to the axis ("activeconductors") may be spaced at a distance equal to the pitch distancebetween successive circumferentially disposed magnets attached to theinner and/or outer rotor.

The means for determining the relative positions of the two rotors maycomprise a signal processing means, which may include a peak detector todetect the peak value of emf produced by each conduction means.

Conduction means may be provided at axially opposite ends of the rotorwhereby to measure the skew of the rotor.

The conduction means may be provided by tracks provided on a flexiblesheet base, the flexible sheet base being wrapped around the housing,and being attached thereto, preferably by adhesive.

Preferred embodiments of the invention will now be described by way ofexample only and with reference to the accompanying drawings in which:

FIG. 1 is an axial section through a seal-less pump showing the generalarrangement thereof, to which apparatus according to the invention maybe applied,

FIG. 2 is a diagrammatic perspective view of part of the apparatus ofFIG. 1, showing inner and outer rotors,

FIG. 3 is a view similar to FIG. 2 of an alternative arrangement,

FIG. 4 is a diagrammatic view of a shroud, forming part of the apparatusof FIG. 1 to which a sensor is mounted,

FIG. 5 is a view similar to FIG. 4 showing an alternative arrangement ofsensors,

FIGS. 6 to 9 are part developed views showing various arrangements ofsensors,

FIG. 10 is a part axial section through a pair of adjacent permanentmagnets on the inner and outer rotor and the part of the shroudtherebetween,

FIG. 11 is a diagrammatic transverse view showing the position ofsensors,

FIG. 12 is a mathematical drawing,

FIG. 13 is a block electrical circuit diagram showing the components ofa signal processing apparatus for processing the output signals of thesensors.

FIG. 14 is a view of a sheet comprising the electrical circuitry forforming the sensors,

FIG. 15 shows a front view of a control panel for controlling thebearing measuring apparatus,

FIGS. 16 to 18 are part developed views showing various arrangements ofsensors in which the lead wires are arranged to improve the accuracy ofthe signal, and,

FIGS. 19 and 20 are wave forms produced by the arrangements,respectively, of FIGS. 17 and 18.

Referring to FIG. 1 there is shown an axial section through a seal-lesspump which may include the apparatus of the invention. There is provideda centrifugal pump 10 comprising a casing 11 having an inlet 12 and anoutlet 13 the casing 11 mounting therein an impeller 14. The impeller 14is driven by an inner rotor 16 having an axis 15, the inner rotor 16being mounted within a housing in the form of a generally cylindricalsealing shroud 17. The sealing shroud 17 includes a closed outer end 18and an inner end 19 in the form of a flange sealingly engaged with thecasing 11 of the centrifugal pump 10 whereby the centrifugal pump 10 isentirely sealed. The closed outer end 18 of the shroud 17 mounts a firstcylindrical bearing 21, and its inner end mounts a second cylindricalbearing 22, and the inner rotor 16 is mounted between the bearings21,22.

In addition to the cylindrical bearings 21,22, there is provided mountedin the open inner end 19 of the shroud 17 a main thrust bearing 20 torestrain axial thrust in the inner rotor 16. The radial end faces of thesecond cylindrical bearing 22 provides a second thrust bearing 20.

The inner rotor 16 mounts around its circumference permanent magnets 23(see FIG. 2). As is clear from FIG. 2 the permanent magnets 23 are, whenviewed in axial section, "U" shaped, but extend longitudinally parallelto the axis. The effect, therefore, is to provide North and Southpermanent magnetic poles successively around the circumference of theinner rotor 16. Alternatively the magnet poles may be provided byrectangular (radial) section blocks of rare earth material.

It will be noted that there is only a small clearance between the outersurface of the magnets 23 and the inner cylindrical surface of theshroud 17.

Surrounding the shroud 17 is a cylindrical outer rotor 24 which as canbe seen from FIG. 2 comprises a similar (but oppositely directed) set ofmagnets 26 to the magnets 23, the magnets 23/26 being equal in numberand disposed so that the poles of the magnets 26 may be directedopposite the opposite polarity poles of the magnets 23. The magnets 26are suitably mounted to a shaft 25 and the outer rotor 24 is mountedwithin an outer housing 28, on either an intermediate shaft or, via anadapter, the output drive shaft of a standard electric motor 31. Thepump is thus driven by a drive apparatus comprising the outer rotor 24,the inner rotor 19, the sealing shroud 17, and the outer housing 28. Thestandard electric motor 31 drives the outer rotor 24.

The arrangement thus far described with reference to FIGS. 1 and 2 ofthe drawings comprises a well known type of seal-less pump. In operationthe electric motor 31 rotates the outer rotor 24. The outer ring ofpermanent magnets 26 locks to the inner ring 23 of permanent magnets(poles of opposite polarity in the two sets of magnets being arrangedadjacent one another), the magnetic field between the poles in the tworings of magnets 23,26 passing through the shroud 17. As a result,rotation of the outer rotor 24 is transmitted to the inner rotor 16 andthis drives the impeller 14.

From time to time, it is necessary that the bearings of the inner rotor16 should be checked. The bearings 20,21,22 of the inner rotor 16 canonly be checked by removing the shroud 17. As the shroud 17 seals thepump 10, it is necessary to drain the system to remove the fluid frominside the pumps 10. This may be hazardous in that the seal-less type ofpump is often used for pumping hazardous fluid. It is also expensive inplant downtime and labour costs.

It would therefore be preferable for means to be provided to measure thewear in the bearings within the shroud 17.

As has already been described, there is provided a magnetic fieldbetween the inner and outer ring of magnets 23,26. The field itselfbetween a given pair of magnets on the inner and outer rotors willchange as the distance between the inner and outer rotors changes. Asthe cylindrical bearings 21,22 of the inner rotor 16 wear, the relativeposition of the inner rotor 16 with respect to the outer rotor 24 willvary. Typically, as the cylindrical bearings 21,22 wear, the inner rotor16 will tend to drop (although this may not necessarily be the case, thewear may be in a different plane). This will cause the magnetic fieldbetween the rotors to change.

Of course, the particular value of magnetic flux varies around thecircumference of the inner rotor and is at a maximum where the poles ofthe magnets of the inner and outer rotor are adjacent one another.

We now refer to FIG. 4 which shows, in diagrammatic form, a shroud 17.According to FIG. 4, there is mounted on the outer surface of the shroud17 (although it may be mounted on the inner surface but that wouldneedlessly complicate matters or, if the shroud is of layered or plasticconstruction, within the material of the shroud, or may be printed ontothe surface of the shroud) a sensor 36 for detecting the magnetic fieldat the surface of the shroud 17. The sensor 36 may comprise a conductivewire which extends parallel to the axis of the apparatus from the innerend 19 to the outer end 18. However because it is within the outer rotor24, it is necessary for the conductive wire of sensor 36 to return backto the inner end 19 and so, in accordance with the arrangement in FIG.4, we provide the sensor 36 in the form of a square or rectangular loop37 of conductive wire. There is provided an input lead wire 38 and anoutput lead wire 39 which are arranged to be close to one another andparallel, and extend from the inner end 19 of the shroud 17 to thesensor loop 37. The sensor loop 37 has two active conductors 41,42 whichextend parallel to the axis 15, and two connector wires 43,44 whichextend circumferentially. The inner connector wire portion 43 is brokenand connected with the input and output lead wires 38,39. The activeconductors 41,42 form sensing wires since, as will be understood, as theinner and outer rotors 16,24 rotate the lines of magnetic field will cutthe active conductors 41,42 and provide an emf therein, the value ofwhich depends, at any one time, on the speed of rotation and themagnetic field.

FIGS. 8 and 9 show developed diagrams showing two possible relativedisposition between the loop 37 and adjacent magnetic poles (N and S) ofthe magnets 23 or 26. In FIG. 8, it will be seen that the activeconductors 41,42 are longer than the axial length l of the magneticpoles, and in FIG. 9 they are shorter. It will also be observed that inboth cases the circumferential connector wires 43,44 are of a lengthwhich equals the pitch length p so that the two active conductors 41,42are spaced apart by the pitch length p and are therefore disposed undercorresponding parts of adjacent poles N and S.

Because the active conductors 41,42 are spaced by p, the pitch distance,the maximum signal possible is obtained since both active conductors41,42 are directly under their respective but opposite magnetic poles atthe position of maximum field strength. Each active conductor 41,42 cutsthe magnetic flux of the rotating magnetic field to produce induced emf,emf in each of the active conductors 41,42 adding by virtue of the factthat oppositely directed active conductors 41,42 are passing in the samedirection through oppositely disposed magnetic fields, as has alreadybeen described, the flux varies, being at a maximum value where twopoles are immediately adjacent one another. Thus as the rotors rotate,the emf produced by the active conductors 41,42 will vary in a cyclicmanner as each pair of poles passes the active conductors 41,42, thepeak value of the (emf_(peak)) produced providing a measure of themaximum magnetic flux between the poles.

The input and output lead wires 38,39, by virtue of being placed closeto one another, are situated in the same magnetic field and since theyare disposed oppositely to one another, the emf producing effect iscancelled out in the two lead wires 38,39.

The spacing between the inner and outer rotors 16,24, is a function ofwear of the cylindrical bearings 21,22. (ignoring, for the moment, wearin the electric motor bearings 27 which mount the outer rotor and aresubjected to much less wear than bearings 21,22). As the cylindricalbearings 21,22 of the inner rotor 16 wear and the rotor moves (normally)downwardly, the peak value of magnetic field and hence the flux willchange at different points around the surface of the shroud 17. If, forexample, the inner rotor 16 effectively moves downwardly as thecylindrical bearings 21,22 wear, then by placing a sensor 36 at the top(or bottom) of the shroud 17, the value of emf_(peak) of the sensor 36will change with time as the bearings wear.

Of course, as will be understood, the induced emf_(peak) in each activeconductor 41,42 is also proportional to the length 1 of the activeconductor and as illustrated in FIGS. 8 and 9, should be either slightlylonger or slightly shorter than the axial length of the magnetic polesto allow for movement of the magnetic poles axially.

We will now describe the operation of the arrangement thus far describedwith reference to FIG. 1,2,4,8 and 9. As the rotors 16,24 rotate thensuccessive pairs of poles in the inner and outer rotors will pass overthe sensor 36. The flux will vary as the rotors rotate from a maximumwhen the poles are situated as shown in FIGS. 8 and 9, that is with themiddle of each pair of poles in inner and outer rotors directly overeach active conductor 41,42, to a minimum when the gaps between eachpair of poles on the outer rotor (or inner rotor) overlie the activeconductors 41,42. Effectively therefore a cyclic emf is produced ofgenerally sinusoidal form. By detecting the peak value of emf, one canprovide an output signal which is dependent upon the peak value ofmagnetic flux between the magnets 23,26.

As the bearings wear, the inner and outer rotors move with respective toone another in a radial direction. If the sensor is placed in positionin which the inner and outer rotors are moving relatively towards oneanother, as the bearings wear, then the peak value of the detectedsignal emf_(peak) will increase because the maximum cyclic value ofmagnetic flux will increase. Conversely, if the sensor 36 is placed at180° to that position, then the magnets of the inner and outer rotorswill move apart as the bearings wear and thus the peak cyclic magneticfield will decrease and the emf_(peak) output signal will reduce. Inprinciple, therefore, one can measure the amount of wear in the bearingsof the inner rotor by this method, without the necessity to strip thepump to view the bearings themselves.

For the measurement of radial wear, the sensitivity of the sensor isindependent of the length of the sensor 36, since for radialdisplacement of the inner rotor 16 it is the magnetic flux density whichis changing.

The geometry shown in FIGS. 8 or 9 would give the maximum signal andtherefore the maximum signal to noise ratio, since the error signal inthe lead wires 38,39 is constant, thus improving the accuracy of theradial wear measurement.

We have described the arrangement utilising a single sensor 36 which,for example, may be situated at the top or the bottom of the housing.

In a preferred arrangement, however, in order to be able to determineany radial wear of the cylindrical bearings 21,22 in any radial positionaround the axis 15, we prefer to provide three or more, preferably foursensors similar to the sensor 36 spaced at 90° intervals around thecircumference of the shroud 17. Typically, the sensors 36 may be at thetop, bottom and two opposite sides of the shroud 17. In this way, bymeasuring the emf_(peak) of all of the sensors 36, we can determine anyradial wear of the cylindrical bearings 21,22.

We will refer to the two side sensors as 36A,36B and the top sensor as36C, the bottom sensor as 36D. These are shown in FIG. 11 which shows adiagrammatic end view.

FIG. 11 shows the physical arrangement of the sensor array in relationto the outer and inner rotors 16,24. For simplicity a physicallysymmetrical model is shown in which all initial air gaps a, b, c, and dbetween the inner rotor and the shroud adjacent respectively sensors36A,B,C,D are equal and equal I. The output voltages of sensors36A,B,C,D (i.e. emf_(peak)) respectively are V_(A), V_(B), V_(C) andV_(D) and are assumed to be initially equal. Wear vertically downwardswill cause the sensor voltages V_(C) and V_(D) to decrease and increaserespectively, due to the respective increasing and decreasing gapbetween the rotors. Sensors voltages V_(A) and V_(B) will both decreaseby an equal amount but less than the decrease in V_(C). Conversely wearvertically upwards will cause V_(C) to increase, V_(D) to decrease, andV_(A) and V_(B) to both decrease. Wear in the horizontal direction canbe determined by similar reasoning.

In general the direction of radial wear will not be along either of thesensor axes 36A/36B or 36C/36D and to measure the wear requires ageometrical model as shown in FIG. 12 which is a geometric equivalentrepresentation of FIG. 11. It can be shown that for any general point P(the position of the axis of inner rotor 16 after wear in the bearings21,22) the vertical and horizontal components x and y, from the originalaxis are given by: ##EQU1## from which can be deduced the wear magnituder and angle Θ since: ##EQU2##

Now the relationship between the sensor voltages and their correspondingair gap is given by: ##EQU3##

Where V is the initial voltage in a physically symmetrical modelcorresponding to the initial gap I and K_(r) is a constant.

For a given pump the values of I and K_(r) will be known, hence thevalues of x,y,r and Θ can be calculated.

The sensor voltages V_(A), V_(B), V_(C), V_(D) are all proportional tospeed and any other changes will introduce proportional errors in thecalculated values of r. However, the voltage V can be expressed in termsof V_(A), V_(B), V_(C), V_(D) and the machine constants, which isproportional to the speed and independent of radial wear.

It we divide the term for r by the speed term, which is proportional tospeed but independent of radial displacement, then we have an expressionfor radial wear which is independent of speed. Hence: ##EQU4##

Where Ks is a constant

We have so far confined the description to the use of a plurality ofsensors spaced at various radial points around the axis to measure thewear in the cylindrical bearings 21,22. It is also useful to measurewear in the two thrust bearings 20 and to do this it is necessary tomeasure the axial movement of the inner rotor 16.

This may be carried out by providing two sets of sensors adjacentaxially opposite ends of the magnets 26. This arrangement is shown inFIGS. 5,6 and 7. In this arrangement, there is provided a first set ofsensors 46 at one axial end of the magnets 26, and a second set ofsensors 47 at the opposite axial end of the magnets 26. Each set ofsensors 46,47 may comprise a single sensor but will more usuallycomprise the four sensors already referred to. Thus a set of eightsensors in all will be provided.

In this case, each sensor 46 comprises two active conductors 41,42spaced apart by the pitch distance p, the length of the activeconductors passing beyond one end of the magnet 23. Similarly the othersensor 47 is arranged straddling the opposite end of the magnet 23. FIG.7 shows a somewhat similar arrangement but in this case adjacent sensors46A,46B in a first set of sensors are disposed in axially differentpositions, one straddling the end of the magnet 23 and the other beinginside the length of the magnet 23. A similar arrangement applies tosensors 47A, 47B.

FIG. 10 is a part diagrammatic axial section showing the arrangement ofFIG. 6. It shows the relative dispositions of the magnets 23,26, theshroud 17, and the sensors 46,47.

Consider now the effect of axial displacement of the inner rotor 16relative to the shroud 17 on the two sets of sensors, 46,47 as shown inFIGS. 6 and 10. Axial displacement of the inner rotor 16 will result inthe output emf_(peak) of one set of sensors 46 or 47 increasing and theother decreasing. If V₁ and V₂ are the corresponding sensor voltagesthen the displacement is proportional to V₁ -V₂.

However V₁ and V₂ are both proportional to speed, and therefore anyspeed changes will alter the term V₁ -V₂. If we divide this term by V₁+V₂, which is proportional to speed but independent of axialdisplacement, then we have an expression for axial wear which isindependent of speed. Hence:

Where K_(a) is a constant. ##EQU5##

Furthermore, in the arrangement of FIG. 6, in which there are provided,for example, four sensors in each of set of sensors 46,47 we may, bymeasuring simultaneously any offset movement of the part of the innerrotor detected by the two sets of sensors 46,47, detect any skew of theinner rotor 16. This may be calculated by the same method as isdescribed with reference to FIG. 12 but in respect of the two sets ofsensors 46,47, and comparing them.

We now consider the special case of a locked rotor. This abnormalbehaviour can occur for a number of reasons, and early detection andaction is necessary to prevent damage. With a stationary inner rotor 16severe distortion of the magnetic field pattern takes place andmanifests itself as a significant reduction in sensor signal levels,much larger than any reduction due to bearing wear or small speeddeviations about the nominal. Since for a given pump running at a givennominal speed the average values of signal levels will be known, largesignal changes can easily be discriminated from the small changes due tobearing wear, or the complete absence of a signal due to an open circuitsensor, in order to detect a locked rotor condition.

The form in which the sensors 36 or 46, 47 are provided can vary. Theconductive portions comprising the input and output lead wires, activeconductors and connector wires may be provided by suitably shapedlengths of wire mounted to the shroud 17, but we prefer to provide themin the form of conductive tracks on a flexible sheet substrate. FIG. 14shows laid out flat a sheet 51 of non-conductive flexible substrate onwhich are laid in the form of conductive copper tracks, sets of sensors46,47. The sheet 51, in use, is wrapped around the outer surface of theshroud 17, the width of the sheet 51 equalling the outer circumferenceof the shroud 17, and the length of the sheet equalling the axial lengthof the shroud 17. A tongue 52 of the sheet 51 may provide output leadwires for all of the sensors of the sets of sensors 46,47.

We now refer to FIG. 13 which shows a block diagram of the electronicapparatus for dealing with the output signals from the sensors. Onesensor 46A,46B from each set of sensors 46,47 is shown. The wires fromeach sensor are connected to a respective filter 53, each filter 53being connected to a multiplexer 54. The multiplexer 54 under control ofa SELECT signal selects the output signal of one of the filters andpasses this to an amplifier 56, the output of the amplifier 56 beingpassed to a peak detector 57. The output of the peak detector 57 ispassed to an analog to digital converter 58. A microprocessor 59 isprovided having connections to the peak detector 57 and to the analog todigital converter 58, and controlling (by the SELECT signal) themultiplexer 54. The microprocessor 59 also drives a display 61 and iscontrolled by input control signals CTRL.

FIG. 15 shows a front view of the control panel for the apparatus thusfar described. The control panel 63 comprises a bearing conditionindicator 66 in the form of ten lamps. The first three lamps 67 arelabelled "AS NEW", the middle four lamps 68 are labelled "INSPECT" andthe right hand set of three lamps 69 indicate "REPLACE", the lamps maybe of different colours.

There are five further lamps as follows:

lamp 72 labelled "LOCKED ROTOR"

lamp 72 labelled "BEARING WEAR LIMIT"

lamp 74 labelled "SENSOR FAILED"

lamp 75 labelled "MONITORED FAILED"

lamp 76 labelled "MAINS"

There is provided a key switch 77 and buttons 78 labelled "ACTIVATE" and79 labelled "ALARM CANCEL". An audible alarm is provided behind slots81. In use of the control panel, when the pump is first installed withnew bearings, it must be initially calibrated. The key switch 77 isturned to the right hand position (labelled "BEARING CHANGE") and theACTIVATE button 78 is compressed. It is known that at this point thebearings are in order and therefore the initial signal values arerecorded and memorised by the electronic circuit as the pump rotates.

After this initial recording of the output signals, the key switchreturns to the position shown in FIG. 15. At this stage, only one of thelamps 67 "AS NEW" should be illuminated. As the bearings wear with time,the signal values will change and this can be indicated by an increasein the number of lamps 66 illuminated. As the lamps 68 begin to beilluminated, the bearings should be inspected and certainly by the timethe lamps 68 are illuminated the bearings should be replaced. As onereaches the bearing condition where the bearing should be replaced, thealarm 73 may be operated.

Similarly, if any of the sensors should fail, as indicated by the outputsignal from a particular sensor, then the alarm may be caused tooperate. The lamp 74 "SENSOR FAIL" will also be illuminated so that itis obvious to the operator what the problem is.

Similarly, if the rotor locks, then the lamp 72 will be illuminated andthe alarm operated. If the monitor fails, then the lamp 75 "MONITORFAIL" will be illuminated and the alarm indicated.

An additional facility is also provided to effect automaticrecalibration of the system when the button 78 is pressed and the keyswitch 77 is in its left hand position. This is necessary onsynchronously coupled pump drives (as illustrated in FIG. 2) after theyhave been dismantled and reassembled (for example to free a lockedrotor) but without replacing the bearings. Since there are a number ofradial positions in which the two rotors can be magnetically locked itis likely that during maintenance the relative rotor positions will bealtered and this will modify the sensor voltages by virtue of themechanical and magnetic tolerances of the magnets. This will have theeffect of increasing or decreasing the sensor voltages by a constantamount and not by a proportional amount and the mathematical models usedby the software for measuring axial and radial wear will be in error.When a recalibration is initiated as above, the initial storedparameters are modified such that the correct values (i.e. those alsostored prior to disassembly) are obtained. This facility is unnecessaryfor a torque ring drive as shown in FIG. 3. It will be understood thatby use of key switch 77 the apparatus can only be recalibrated or abearing change noted by an authorised operator.

The electronics circuit may continuously or at intervals cross check thesignal for each sensor against the stored initial value of signal forthat sensor and from this difference in values determine the movement ofthe inner rotor and hence the bearing wear.

We now consider some details of the set-up and reset aspects of theapparatus. Due to mechanical tolerances, the initial values of sensorvoltages, i.e. the sensor voltages on a pump fitted with new bearings,will not be equal, and therefore give rise to initial axial and radialoffsets.

The initial axial offset is caused by asymmetrical axial alignment ofthe inner and outer rotors and the shroud, due to manufacturingtolerances; and the accuracy to which the sensors can be attached to theshroud 17. The permissible manufacturing tolerance of the apparatus isthe dominant cause of misalignment, although it can be regarded asremaining constant for a given pump throughout its lifetime, With thepermissible tolerances, the difference between the worst case extremesof the axial displacement is comparable with that of the length of eachsensor itself. Under such worst case conditions, the sensors could betotally misaligned with the rotors on different pumps, and this wouldseverely affect the sensitivity of the sensor and the accuracy of anymeasurement.

The positional errors in attaching the sensors to the shroud alsointroduces a degree of initial misalignment. This can also be regardedas constant for a given pump and time-invariant.

To compensate for the foregoing axial offsets, a plurality of staggeredaxial sensors may be provided as shown in FIG. 7, and the optimum pairselected either manually or automatically in order to maximise thesensor sensitivity and the accuracy of measurement. Differentarrangements of sensors may be provided for different designs of pump.

The initial axial position of the inner rotors is also subject to themanufacturing tolerance of the bearings supporting the rotor shaft evenwhen new.

There is initial axial freedom for the shaft within defined limits knownas the end float. In practice the shaft will normally assume a positionto the left in FIG. 1, by virtue of the hydraulic thrust force on theimpeller. By measuring and storing the initial axial offset value andsubtracting the initial value from the measured value, a true measure ofaxial displacement can be obtained.

Initial radial offsets are due mainly to the non-concentricity betweenthe rotor and the shroud, and which for a given pump can be regarded asconstant. By measuring and storing the initial voltages we can calculatethe initial value of the co-ordinates x and y. By subtracting theinitial values from the measured values we can obtain a true measure ofradial displacement.

We have described the arrangement with respect to a drive apparatushaving an inner and outer ring of magnets as shown in FIG. 2. FIG. 3shows an alternative arrangement to which the invention may be appliedin which only the outer rotor includes permanent magnets, the innerrotor including a so called torque ring in which, by virtue of inducededdy current, a magnetic field is induced in the inner rotor without theprovision of permanent magnets in the inner rotor.

We now refer to the arrangements of FIGS. 16 to 20.

FIG. 16 shows a typical pair of sensor loops 80,81 for measuring radialand axial wear, although in practice four such pairs are required. Eachsensor loop has a pair of lead wires 82,83 (connected to loop 80) and84,85 (connected to loop 81). The lead wires 84,85 will be subject to aninduced emf by virtue of the changing magnetic field. The lead wires82,83 are of less relevance because they are in the low field strengtharea.

The emf produced by each lead wire 84,85 tends to cancel outparticularly as they are arranged to be as close as possible and arearranged centrally with respect to the sensor loop 81. If the individualwires of the lead wires 84,85 were coincident in space (theoreticallypossible only) there will be no resultant induced emf in the wires sincethe individual emfs (as shown by the arrows in FIG. 16) would not onlybe equal but also oppose each other and therefore give perfectcancellation.

In practice the individual wires are spatially disposed, and due to thenon-uniform and continually changing magnetic field a small resultantnoise emf will be induced and this is added to the signal from thesensor loop 81 thus giving an error component.

Although this noise signal is minimal at the time the emf in the sensorloop 81 is at a maximum by virtue of its central positioning, and thepeak detector samples at this point to maximise the signal-to-noiseratio, the error component is still finite. The error signals areproportional to the length of the conductors in the magnetic field andas can be seen from FIG. 16 the lead wire length of sensor loop 81 issignificantly greater than that of sensor 80. The result of this is theaccuracy of radial wear calculation using the signal from sensors withlong lead wires is lower than that obtainable from sensors with a shortlead wires, due to the higher error component.

FIG. 17 shows an arrangement of lead wires 84,85 and sensor loop 81 inwhich the lead wire 84 is arranged to cross over the lead wire 85 bymeans of a cross over wire 86, then extends back parallel to wire 85 bymeans of a lead wire 87 towards the sensor loop 81, and then passes backparallel to the lead wire 87 by means of a lead wire 88, the lead wires87 and 88 forming a side loop 89. Since the side loop 89 is of the samelength as the sensor lead wires 84,85, and also in close proximity tothem, the induced emfs in the lead wires 84,85,87,88 are similar. Thereis a slight phase shift between the induced emfs from 84,85 and 87,88 byvirtue of the physical disposition of the conductors. However their peakamplitudes are the same. By connecting them as shown the induced emfstend to cancel each other out giving a much lower overall residual noisecomponent.

FIG. 19 shows the noise component wave form in both the lead wires 84,85and side loop 89 and their resultant as seen by the monitor.

FIG. 18 shows an improved arrangement in which there are provided twoside loops 90,91 on either side of the lead wires 84,85 and connectedsuch that the side loop emfs add but subtract from the emf produced bythe lead wires 84,85 with even better noise reduction. As can be seenthe length of each side loop must be half of that of the lead wires84,85 since there are two loops and their voltages add. The wave formsare shown in FIG. 20.

The above arrangement can be extended to any number of side loops of thecorrect lengths and suitably connected to give even better noisereduction. In practice it has been found that two side loops is a goodcompromise between noise reduction and sensor wiring complexity.

We claim:
 1. An apparatus for measuring change of position of a rotor ina drive system comprising a first rotor adapted to be driven by a drivemeans and a second rotor generally concentrically mounted with respectto said first rotor, said first and second rotors being magneticallycoupled together, whereby said second rotor is driven by said firstrotor, said apparatus comprising:a conduction means stationarily mountedbetween said rotors for producing emf (electro motive force) as saidrotors rotate, means for measuring the emf produced in said conductionmeans as said rotors rotate, and means for determining, from saidmeasurement, the relative position of the two rotors and/or the positionof one of the rotors.
 2. The apparatus as claimed in claim 1, whereinthe conduction means is mounted on a stationary housing between thedriven and drive rotor.
 3. The apparatus as claimed in claim 1 or 2,wherein the drive system is a seal-less pump including a sealing shroudbetween the drive rotor and the driven rotor, said conduction meansbeing mounted on or forming part of said sealing shroud.
 4. Theapparatus as claimed in claim 3, wherein the conduction means isprovided by conducting tracks on a flexible sheet base, the flexiblesheet base being wrapped around the housing and being attached thereto.5. The apparatus as claimed in claim 1, wherein two conduction means areprovided at 180° intervals around the axis of the housing.
 6. Theapparatus as claimed in claim 5 wherein one conduction means is at thetop of the housing and one is at the bottom of the housing.
 7. Theapparatus as claimed in claim 1, wherein four conduction means areprovided spaced at substantially 90° intervals around the axis of therotors.
 8. The apparatus as claimed in claim 1 wherein the conductionmeans comprises one or more loops including lengths extendingsubstantially parallel to the axis and the other lengths extendingcircumferentially around the housing.
 9. The apparatus as claimed inclaim 8 wherein the lengths extending parallel to the axis are spaced ata distance equal to the pitch distance between successivecircumferentially disposed magnets attached to the in and/or outerrotor.
 10. The apparatus as claimed in claim 1, wherein the means fordetermining the relative positions of the two rotors comprises a signalprocessing means which includes a peak detector to detect the peak valueof emf produced by each conduction means.
 11. The apparatus as claimedin claim 1, wherein the conduction means is provided at axially oppositeends of the rotor whereby to measure the skew of the rotor.
 12. Theapparatus as claimed in claim 7, wherein said one or more loops areconnected to said means for measuring the emf by means of lead wires, atleast one of said lead wires including a side loop.